As more light-emitting diode (LED) illumination systems penetrate the general lighting market, demands on performance, including efficiency, increase. LED-based illumination systems are not only expected to provide higher quality light over a longer operational life, but these LED-based illumination systems are also expected to do so efficiently across all applications. While much research and development in this area has been directed toward new LED materials, manufacturing equipment and processes, the converters or drivers that power the LED-based illumination systems significantly influence the overall efficiency of the systems.
LEDs used for illumination applications often require a regulated current at a voltage above a minimum level to deliver consistent (i.e., without periodic modulation or flicker) light output. The LED must also continue to operate without flicker even when disruptions or dropouts in the input power occur. To accommodate these requirements, conventional converter topologies deploy a two-stage arrangement having a boost converter followed by a buck converter. The boost converter is used since it can be designed to achieve a near-unity power factor (PF). To maintain this high PF, however, the boost converter output voltage, or intermediate voltage, must be at least twice the root mean square (RMS) of the input voltage, requiring the second step-down buck converter stage. The buck converter reduces the intermediate voltage to that required by the LED and also delivers a highly regulated, low ripple current into the LED to minimize flicker.
In this arrangement, the higher the intermediate voltage, the smaller the storage capacitor across the output of the boost converter required to hold the LED voltage through any dropout periods. However, as this voltage is increased, the converter becomes less efficient. In very small LED lamps such as the MR16, this leads to a very challenging tradeoff between efficiency, cost, and lamp size. Typical efficiencies for boost and buck converters with 3:1 transformation ratios are around 90%. Thus, the net efficiency of this combination is the product of the efficiency of the two stages, or approximately 81% (90%×90%).
Furthermore, the buck converter requires that the intermediate voltage (the voltage output of the boost stage), be significantly higher than the minimum operational LED voltage. This results in a significant amount of energy stored in the boost capacitor that cannot be extracted by the buck converter to hold up the LED. This again leads to having to size the energy storage components of the converters larger than desired.
Applicant therefore identifies a need to power LEDs without flicker or dropout, and yet provide high efficiency. The present invention fulfills this need among others.